Devices capable of detecting electromagnetic radiation, commonly referred to as photodetectors, have many applications in modern technology. These devices typically produce an electrical signal in response to the radiation and are used in such diverse applications as solar cells, infrared sensors, and photodetectors in optical communications systems. The latter application has become increasingly important with the development of low loss silica-based optical fibers. In the optical communications systems presently contemplated, such fibers optically couple light sources ansd photodetectors.
As the desired data rates for such optical communications systems increase, as well as for other applications, there has been an increase in demand for still faster photodetectors. Several photodetectors have been developed that are capable of detecting approximately picosecond pulses. For example, Applied Physics Letters, 37, pp. 371-373, Aug. 15, 1980, discloses a high speed photodetector using an amorphous semiconductor such as, for example, silicon.
Various approaches, for example, new structures or compositions, have been tried in attempts to improve photodetector performance. With the development of new crystal growth techniques, such as molecular beam epitaxy (MBE), that permit precise compositional control and variation, additional possibilities for improving photodetector performance by exploiting these approaches have opened. For example, it is now possible to grow semiconductor materials that have both compositional and bandgap variations in the growth direction. It is also possible to grow semiconductor structures that have abrupt composition, including doping, variations. Several photodetectors have been developed that use the capability that MBE has of growing such materials and structures. For example, it has been found that the ratio of the ionization coefficients of electrons to holes can be substantially altered in Group III-V compound semiconductors by compositionally grading the semiconductor. The asymmetry of the ionization energies and the quasi-electric fields that the electrons and holes see is produced by the strong bandgap grading that results from the compositional grading. Another device using compositional variation is the multistage graded bandgap avalanche photodetector, commonly referred to as a "staircase APD", which might be viewed as the solid state analog of a photomultiplier. See, Electron Device Letters, EDL-3, pp. 71-73, March 1982. Graded bandgap materials have been used in other opto-electronic devices, such as solar cells, that were fabricated by other crystal growth techniques such as liquid phase epitaxy (LPE). See, for example, Applied Physics Letters, 30, pp. 492-493, May 1, 1977. The device disclosed had a graded bandgap p-type Ga.sub.1-x Al.sub.x As layer.
The preceding discussion should not give the impression that the use of graded bandgap materials is limited to opto-electronic devices as the use of graded bandgap materials in devices other than photodetectors has long been known. For example, Kroemer proposed in RCA Review, 18, pp. 332-342, September 1957, the use of a graded bandgap base region in a transistor. The effect of the graded bandgap base region is to produce a quasi-electric field which reduces the base transit time which is otherwise normally diffusion limited.